Recent years have seen development of biometric authentication technology for determining whether or not to authenticate an individual, by using biometric information such as a fingerprint and a finger vein pattern. The biometric authentication technology is widely used from large-scale systems with a number of registered users such as an access control system, a border control system, and a system using national identification numbers, to devices used by particular individuals such as a computer and a mobile terminal.
For example, when a fingerprint of any of finger is used as biometric information, a biometric authentication apparatus acquires a biometric image representing the fingerprint as an input biometric image. Then, the biometric authentication apparatus matches the input biometric information, which is a user's fingerprint represented in the input biometric image, with registered biometric information, which is a fingerprint stored in advance in the form of a biometric image for each registered user. Based on a result of the matching process, when determining that the input biometric information matches the registered biometric information, the biometric authentication apparatus authenticates the user as being a legitimate registered user. The biometric authentication apparatus then permits the authenticated user to use an apparatus in which the biometric authentication apparatus is incorporated or other apparatus connected to the biometric authentication apparatus.
Some biometric authentication apparatuses include a sensor that is large enough to be able to acquire biometric information of each of a plurality of fingers at a time. A biometric authentication apparatus using such a sensor is capable of using biometric information of a plurality of fingers. However, in some cases, a degree of spread between fingers held to the sensor when a user registers biometric information of fingers on the biometric authentication apparatus (hereinafter referred to simply as “at the time of registration”) is different from a degree of spread between fingers held to the sensor when the user asks for authentication (hereinafter referred to as “at the time of matching”). In such a case, for example, a feature of each finger extracted from a biometric image generated at the time of matching may be different from a feature of each finger extracted from a biometric image generated at the time of registration, consequently decreasing an authentication rate.
Meanwhile, techniques have been proposed for detecting or correcting difference in the rotation direction between biometric information of a finger on a biometric image acquired at the time of registration and biometric information of a finger on a biometric image acquired at the time of matching, by analyzing the biometric images (refer to, for example, Japanese Laid-open Patent Publication No. 2004-246459, Japanese Laid-open Patent Publication No. 2005-25414, Japanese Laid-open Patent Publication No. 2002-117405, and Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. H11-514771).
For example, Japanese Laid-open Patent Publication No. 2004-246459 discloses a method of calculating an angle of rotational difference between fingerprint images. In this method, an amount of positional difference in each of the x direction and the y direction between two fingerprint images to be matched is obtained, and, after determining the center of one of the fingerprint images, the center of the other fingerprint image is corrected using the obtained amount of difference. Then, in this method, spectral patterns in the rotation direction are obtained from one fingerprint image and the other fingerprint image corrected, and matching is performed between the spectral patterns in the rotation direction by moving one of the patterns, thereby an angle of rotational difference between the two fingerprint images is obtained.
Japanese Laid-open Patent Publication No. 2005-25414 discloses a technique of successively reading, for every certain angle, a fingerprint image for matching, selecting an image having an angle of a finger approximately the same as that of a registered image stored in advance, from the plurality of images for matching read, and then performing matching with a registered image.
Japanese Laid-open Patent Publication No. 2002-117405 discloses a palm-shape authentication method of detecting a rotation angle of a palm from palm-shape image data, correcting positional difference with a rotation angle of a registered palm in a registered palm-shape image being registered in advance by detecting a palm shape of a user, and then, performing matching between the palm-shape image and the registered palm-shape image.
Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. H11-514771 discloses a fingerprint-image identification method of obtaining a longitudinal axis of each of at least two fingers positioned side by side by detecting a centroidal line of a fingerprint image, and obtaining angles of orientation of the centroidal lines and difference between the angles of orientation.